With increasing demands for higher performance of machinery and the conservation of material and energy used to make machinery there are demands for higher performances of the various components of the machinery. In respect to bolts these demands result in pressures to tighten bolts to a higher percentage of their tensile strength so that fewer or smaller bolts will accomplish the required job. In response to these pressures more and more bolt users are tightening bolts until yielding occurs, on the theory that this produces the maximum feasible level of tension which it is safe to put in the bolt.
As part of this movement towards higher bolt tensions, recently there have been announcements of two wrenching systems which have the capability to tighten bolts until yielding has just occured, (Machine Design, Vol. 47, No. 2, Jan. 23, 1975, page 44, and Design News, Vol. 30, No. 17, Sept. 8, 1975, page 57, both herein incorporated by reference). These systems incorporate means for measuring the wrenching torque and rotation of the nut (or bolt, if the bolt head is being wrenched) and they determine when yielding occurs, and hence the wrenching should be stopped, by monitoring when the slope of the torque-rotation curve drops appreciably, this drop in slope being characteristic of the bolt as it passes through the yield point from the region of elastic behavior to that of plastic behavior. In my copending application, "Process for the Pre-use Work-Hardening of Bolts and Bolts Obtained Thereby," Ser. No. 664,574, herein incorporated by reference, I describe a simple and inexpensive method of treating bolts in such a way that all bolts so treated will have the same yield point (proof load) and thus all will have the same tension (preload) when tightened to their yield point.
While the combination of these new wrenching systems and the method of pre-use treatment put forward in my copending application leads to high and uniform in-place bolt tensions, there is one problem with tightening bolts until they reach their yield point. A bolt tightened to this condition will yield further if, under service conditions, external forces put any additional increment of load on the bolt. The bolt extension accompanying this yielding may cause the bolted joint to malfunction immediately, such as, for example, to cause loss of pressure in a pipe flange connection. Further, when, subsequently, the external forces change again so as to remove the increment of load the bolt will be longer by the amount of the further yielding and its clamping tension will be reduced proportional to the amount of this additional elongation. In short, the problem with tightening a bolt to its yield point is that the bolt then has no built-in overload protection against further yielding or against loss of preload.